EP3012337B1

EP3012337B1 - Hot-forged ti-al-based alloy and method for producing same

Info

Publication number: EP3012337B1
Application number: EP14814461.1A
Authority: EP
Inventors: Toshimitsu Tetsui
Original assignee: National Institute for Materials Science
Current assignee: National Institute for Materials Science
Priority date: 2013-06-19
Filing date: 2014-06-02
Publication date: 2018-04-25
Anticipated expiration: 2034-06-02
Also published as: US10208360B2; EP3012337A1; US20160145703A1; EP3012337A4; WO2014203714A1

Description

Technical Field

The present invention relates to a TiAl-based alloy to be suitably used for a rotor blade of a gas turbine for power generation, a gas turbine for aircraft, or the like, and specifically, to a hot-forged TiAl-based alloy in which hot forgeability is excellent, strength is high at a high temperature, and ductility is also excellent in a room temperature. In addition, the present invention relates to a method for producing the hot-forged TiAl-based alloy.

Background Art

Recently, as materials used for a rotor blade of various turbines, TiAl-based alloys, being lightweight and having excellent heat resistance, have attracted attention. Particularly, in the case of a large rotatable rotor blade, as the constituent member of the rotor blade become lighter, the centrifugal stress becomes smaller, which enables improvement in the maximum engine speed, an increase in area of the rotor blade, and a reduction in the load stress applied to a disk portion of the rotor blade and is very beneficial to the increase in efficiency of the entire apparatus.
This TiAl-based alloy is an alloy composed mainly of TiAl or Ti₃Al, which is an intermetallic compound having excellent high-temperature strength, and the alloy is excellent in heat resistance as described above. The TiAl alloy, which is a lightweight heat resistance alloy, is used as a casting material and a forged material.
The casting material has a perfect lamellar structure laminated with a α2-phase and a γ-phase which are excellent in high-temperature strength, but there is a problem that room-temperature ductility is deficient because forgeability is poor and a crystal grain is coarsened. Therefore, for example, a technique is proposed in Patent Literatures 1 and 2, in which a TiAl-based alloy material as a hot forging material having a predetermined composition is held in an equilibrium temperature range of (α+β)-phase and is then subjected to plastic working, thereby eliminating casting defects and fining a structure by a synergistic effect of working distortion and phase transformation. Moreover, thereafter, the hot-forged TiAl-based alloy material is held in an equilibrium temperature range of (α+β)-phase, (α+β+γ)-phase, or (β+γ)-phase, an area fraction of lamella grain and β-phase or a grain size of the lamella grain is controlled, and thus the TiAl-based alloy having excellent machinability and high-temperature strength can be produced. As a hot working method other than the hot forging, for example, extrusion or rolling-type forging can be used.

Citation List

Patent Literature

Patent Literature 1: JP 4209092 B1
Patent Literature 2: JP 4287991 B1
Patent Literature 3: JP 6-49565 A

Summary of Invention

Technical Problem

However, the case of the casting material described above was not sufficient in view of a general coarseness of the cast structure and improvement in ductility at a room temperature. In particular, with respect to a rotor blade used for an engine for industrial use or the like, foreign matter such as sludge may collide with the rotor blade at the time of operation, or at the time of production of the rotor blade, the blade may be broken due to impact at the time of fixing the blade to an outer periphery of the disk with a hammer. Hence, it becomes necessary to improve ductility or impact properties of the TiAl based alloy. In the casting material of the above conventional technique, however, it was difficult to improve the ductility or the impact properties.
In the case of the casting material, production of small parts such as vehicle parts is relatively easy. However production of large parts has been difficult because castability such as molten-metal flowability of the TiAl-based alloy was generally poor.
On the other hand, isothermal forging is also commonly used as a forging method of the forged material of the TiAl-based alloy, the isothermal forging being characterized in that the mold and the material are held together at a high temperature and are slowly deformed at a constant temperature. With the isothermal forging, however, there are problems in that process costs are very expensive and production of large parts can be difficult because of the limitation of methodology that the mold and the material are heated together.
Meanwhile, with respect to hot forging material in the forged material of the TiAl-based alloy, for example, as disclosed in Patent Literature 3, a β-phase having excellent high temperature deformability (that is, small high-temperature strength) is generated by the addition of a β-stabilization element (Mn, V, Nb, Cr, or the like), and thus so-called hot forging can be performed to cause high-speed deformation as a temperature decreases during the forging. In the hot forged material of the conventional TiAl-based alloy, however, since the β-phase remains in the final product, there were problems in that high-temperature strength was small in a usable state and an available temperature was about 700°C in maximum which was significantly lower than about 850°C which was an available temperature of the casting material.
JP 06340955 describes a TiAl-based alloy comprising 38-53 atom % Al, with ≤ 5% of the Ti substituted by at least one of Mo, V, Mn, Cr, Nb, W and Si. US-6051084 describes TiAl alloys containing Nb and Cr.
The present invention has been made to solve the above problems in the TiAl-based alloy and an object thereof is to provide a TiAl-based alloy which is excellent in hot forgeability as a hot forging material, ductility at a room temperature, and impact properties as well as having excellent high-temperature strength.

Solution to Problem

The present invention provides the TiAl-based alloy of claim 1.
The invention also provides the method for producing this TiAl-based alloy as defined by claim 2. In further aspects, the invention also provides a rotor blade for turbine that uses the TiAl-based alloy material produced in such a manner that an ingot having the composition according to the invention is produced by the production method of the invention, and also provides a turbine that uses said rotor blade.

Advantageous Effects of Invention

According to the present invention, a TiAl-based alloy is provided which is excellent in hot forgeability as a hot forging material, ductility at a room temperature, and impact properties as well as having excellent high-temperature strength.

Brief Description of Drawings

Figs. 1(A) and 1(B) are appearance photographs illustrating a TiAl alloy ingot used in an embodiment of the present invention and Fig. 1(C) is an explanatory view of a hot forging test procedure performed for evaluating hot forgeability.
Fig. 2 is a diagram illustrating a summary of compositions of trial ingots and evaluation test results of the ingots.
Fig. 3 is a diagram illustrating a summary of compositions of trial ingots and evaluation test results of the ingots.
Fig. 4 is an explanatory view illustrating a relation among an alloy element parameter P of a trial ingot, an area ratio of a β-phase existing in a material, which is water-cooled in a condition of 1350°C × 1 h (procedure 2 to be described below), and a forging test result at 1350°C (procedure 3).
Fig. 5 is an explanatory view illustrating a relation among an alloy element parameter P of a trial ingot, an area ratio of a β-phase existing in a material, which is water-cooled in a condition of 1350°C × 1 h (procedure 2), and the presence or absence of a β-phase residue in the case of being subjected to annealing at 0.2°C/min. after being held at 1350°C for 2 h (procedure 4).
Fig. 6 is an appearance photograph of a hot-forged TiAl alloy according to the embodiment of the present invention which is subjected to hot forging at 1350°C.
Fig. 7 is a reflected electron image photograph of a cross-sectional structure for the hot forged TiAl alloy according to the embodiment of the present invention which is heat-treated under appropriate conditions after being subjected to the hot forging.
Fig. 8 is an appearance photograph of a hot-forged TiAl alloy of an alloy 6 as Comparative Example which is subjected to hot forging at 1350°C.
Fig. 9 is a reflected electron image photograph of a cross-sectional structure for the hot-forged TiAl alloy of the alloy 6 as Comparative Example which is heat-treated under appropriate conditions after being subjected to the hot forging.
Fig. 10 is an appearance photograph of a hot-forged TiAl alloy of an alloy 17 as Comparative Example which is subjected to hot forging at 1350°C.
Fig. 11 is a reflected electron image photograph of a cross-sectional structure for the hot-forged TiAl alloy of the alloy 17 as Comparative Example which is heat-treated under appropriate conditions after being subjected to the hot forging.
Fig. 12 is a reflected electron image photograph of a cross-sectional structure for the hot-forged TiAl alloy according to the embodiment of the present invention, as Comparative Example, which is held at 1220°C lower than an appropriate holding temperature in a heat treatment. Other heat treatment conditions are appropriate conditions.
Fig. 13 is a reflected electron image photograph of a cross-sectional structure for the hot-forged TiAl alloy according to the embodiment of the present invention, as Comparative Example, which is held at 1300°C higher than the appropriate holding temperature in a heat treatment. Other heat treatment conditions are appropriate conditions.
Fig. 14 is a reflected electron image photograph of a cross-sectional structure for the hot-forged TiAl alloy according to the embodiment of the present invention, as Comparative Example, which is held for 0.5 hours shorter than the appropriate holding time in a heat treatment. Other heat treatment conditions are appropriate conditions.
Fig. 15 is a reflected electron image photograph of a cross-sectional structure for the hot-forged TiAl alloy according to the embodiment of the present invention, as Comparative Example, which is held for 23 hours longer than the appropriate holding time in a heat treatment. Other heat treatment conditions are appropriate conditions.
Fig. 16 is a reflected electron image photograph of a cross-sectional structure for the hot-forged TiAl alloy according to the embodiment of the present invention, as Comparative Example, which is cooled at 0.7 [°C/min.] slower than the appropriate cooling rate in a heat treatment. Other heat treatment conditions are appropriate conditions.
Fig. 17 is a reflected electron image photograph of a cross-sectional structure for the hot-forged TiAl alloy according to the embodiment of the present invention, as Comparative Example, which is cooled at 15 [°C/min.] faster than the appropriate cooling rate in a heat treatment. Other heat treatment conditions are appropriate conditions.

Description of Embodiments

A TiAl-based alloy according to an embodiment of the invention consists of: 41 to 45 atom% of Al, 7 to 9 atom% of Nb, 0.4 to 4.0 atom% of Cr, 0.3 to 1.0 atom% of Si, and 0.3 to 1.0 atom% of C, and the balance Ti with inevitable impurities. In the TiAl-based alloy, an alloy element parameter P obtained by the following formula is in the composition range of from 1.1 to 2.3, and in a final state after a heat treatment subsequent to hot forging, the TiAl-based alloy has a fine structure in which lamella grains laminated alternately with a Ti₃Al phase (α2-phase) and a TiAl phase (γ-phase) are densely arranged and a β-phase is not included, the lamella grains having an average grain size of 1 to 200 µm: $P = (41 - Al) / 3 + 0.25 Nb + 0.8 Cr - 0.8 Si - 1.7 C$
The other, optional, aspect of the TiAl-based alloy according to the embodiment of the present invention is a TiAl-based alloy in which at least one element selected from the group consisting of W, Mo, B, Hf, Ta, and Zr is further contained in the above TiAl-based alloy to be 0.1 to 3 atom% in total. By the addition of a small amount of these elements, it is possible to increase high-temperature strength, creep strength, and oxidation resistance.
As a method for producing the TiAl-based alloy having the composition, first, an ingot is prepared by dissolution, a process in which the ingot is held at a coexisting temperature range of a hexagonal close-packed structure phase (α-phase) and a body-centered cubic structure phase (β-phase) and is then subjected to hot forging, and a process in which the hot-forged TiAl-based alloy material is held in a temperature range of from 1230°C to 1290°C, which is an α-single phase region, for 1 to 20 hours and is then subjected to a cooling treatment at a rate of from 1 [°C/min.] to 10 [°C/min.].
In the method for producing the TiAl-based alloy according to the embodiment of the present invention, after the structure including the β-phase formed after the hot forging is turned into the α-single phase during the heat treatment in the heat treatment process, and transformation of α → α+γ → α2+γ occurs in the cooling process, that is, the hexagonal close-packed structure phase (α-phase) is transformed into an eutectoid phase of the hexagonal close-packed structure phase (α-phase) and the TiAl phase (y-phase), and is further transformed into an eutectoid phase of the Ti₃Al phase (α2-phase) and the TiAl phase (γ-phase).
A rotor blade for turbine of the present invention is characterized in that the TiAl-based alloy having the above composition is produced by the production method described above.
A gas turbine for power generation, a gas turbine for aircraft, a turbocharger for ship, or a gas turbine or a steam turbine for various industrial machines according to the invention is characterized by using the rotor blade for turbine.
Hereinafter, the reason why the composition and the content of the TiAl-based alloy according to the embodiment of the present invention are limited as described above will described as follows. In the following description, a percentage (%) indicating the content is referred to as atom%.
Aluminum (Al): When the content of Al is in the range of from 41.0 atom% to 45.0 atom%, it is preferred because the β-phase does not exist in a final state after the heat treatment, a perfect lamellar structure laminated with the α2-phase and the γ-phase is obtained, and the hot forgeability is excellent. The excellence in the hot forgeability means that large cracks do not occur even when the hot forging is performed under conditions illustrated in Figs. 1(A) and 1(C) in particular and fine cracks caused by the change in surface structure of oxidation or the like are not included. When the content of Al is less than 41.0 atom%, the hot forgeability is good, but the ratio of the α2-phase becomes too high. Thus, in this case, the ductility may be deteriorated. When the content of Al exceeds 45.0 atom%, the hot forgeability may become poor.
Niobium (Nb): When the content of Nb is in the range of from 7.0 atom% to 9.0 atom%, it is preferred because oxidation resistance is improved. When the content of Nb is less than 7.0 atom%, the effect of improving the oxidation resistance may be insufficient. The content of Nb exceeds 9.0 atom%, problems may arise in that the β-phase remains and the weight increases.
Chromium (Cr): When the content of Cr is in the range of from 0.4 atom% to 4.0 atom%, it is preferred because the hot forgeability is improved. When the content of Cr is less than 0.4 atom%, for example, as indicated in alloys 10 and 23 to be described below, the hot forgeability may be deteriorated. When the content of Cr exceeds 4.0 atom%, the β-phase remains, and the high-temperature strength such as creep strength may be deteriorated.
Silicon (Si): When the content of Si is in the range of from 0.3 atom% to 1.0 atom%, it is preferred because the creep strength is improved. When the content of Si is less than 0.3 atom%, for example, as indicated in an alloy 21 to be described below, the creep strength may not be improved. When the content of Si exceeds 1.0 atom%, the hot forgeability may become poor.
Carbon (C): When the content of C is in the range of from 0.3 atom% to 1.0 atom%, it is preferred because the creep strength is improved. When the content of C is less than 0.3 atom%, for example, as indicated in an alloy 5 to be described below, the creep strength may be insufficient. When the content of C exceeds 1.0 atom%, the hot forgeability may become poor.
In the TiAl-based alloy according to the embodiment of the present invention, the alloy element parameter "P = (41-Al)/3 + 0.25Nb + 0.8Cr - 0.8Si - 1.7C" is preferably in the range of 1.1 atom% to 2.3 atom%. When the alloy element parameter P is less than 1.1 atom%, the hot forgeability may become poor. When the alloy element parameter P exceeds 2.3 atom%, since the β-phase remains even after the heat treatment, the high-temperature strength such as creep strength is deteriorated and thus an available temperature may be lowered.
In the TiAl-based alloy according to the embodiment of the present invention, the crystal grain size of the lamella grain is preferably 1 µm or more and 200 µm or less, and particularly preferably 30 µm or more and 100 µm or less. When the crystal grain size of the lamella grain is 100 µm or less, it is preferred because the room-temperature ductility is ensured. It is industrially very difficult to make the average grain size of the lamella grain to be less than 1 µm, and when the average grain size of the lamella grain is less than 30 µm, production costs may increase or production yield may be reduced. On the other hand, when average grain size exceeds 200 µm, the room-temperature ductility, especially, impact properties may be reduced.
In the method for producing the TiAl-based alloy according to the embodiment of the present invention, the reason why the heat treatment conditions of the forging material are limited as described will be described below. The temperature range in which the hot-forged TiAl-based alloy is held in the equilibrium temperature range of the α-single phase region is preferably from 1230°C to 1290°C. When the temperature range is lower than 1230°C, since it is within the (α+γ) region, the perfect lamellar structure may not be formed after cooling. When the temperature range exceeds 1290°C, since it is within the (α+β) region, the β-phase may remain by the cooling rate after the cooling.
In addition, the time at which the hot-forged TiAl-based alloy material is held within the equilibrium temperature range of the α-single phase region is preferably from one hour to 20 hours. When the holding time is shorter than one hour, the time is too short and thus the α-single phase may not be obtained. When the holding time exceeds 20 hours, the time is too long and thus the crystal grain size of the α-grain (final lamella grain) is coarsened, whereby the ductility or the like may be deteriorated.
Furthermore, the cooling rate after the hot-forged TiAl-based alloy material is held for a predetermined holding time within the equilibrium temperature range of the α-single phase region is preferably from 1 [°C/min.] to 10 [°C/min.]. When the cooling rate is slower than 1 [°C/min.], since the cooling rate is too slow and the gap between the α2-phase and the γ-phase within the lamella grain becomes coarse, the high-temperature strength such as creep strength may be deteriorated. When the cooling rate exceeds 10 [°C/min.], since the cooling rate is too fast and the ratio of the α2-phase is too large, the ductility may be deteriorated.
Specifically, the method for producing the TiAl-based alloy according to the embodiment of the present invention is as follows. First, the ingot having the composition described above is melted. Subsequently, the ingot is subjected to hot forging. That is, similarly with the conventional hot-forged TiAl alloy, after being held in an coexisting region of the α-phase and the β-phase, the ingot is taken out of the furnace and is subjected to the hot forging for working at a high strain rate while being rapidly cooled. In this case, similarly with the hot forged material of the conventional TiAl-based alloy, the hot forgeability can be ensured due to the effect that the β-phase rich in plastic deformability exists. In addition, due to the effect that plastic strain is imparted by the hot forging, the crystal grain size becomes finer.
Subsequently, the hot-forged material is subjected to a heat treatment. In the heat treatment, the material is held for a predetermined time at the α-single phase region, and thus the β-phase existing in the forged material is eliminated and the α-single phase is obtained. Then, by cooling of the forged material at a predetermined rate, transformation of α → α+γ → α2+γ occurs. The crystal grain is not coarsened by optimization of the holding time at the α-region, and it is possible to obtain a perfect lamellar structure laminated with the α2-phase and the γ-phase, which are fine grains and are finally excellent in high-temperature strength and room-temperature ductility, by optimization of the cooling rate. Unlike the hot forging material of the conventional TiAl-based alloy, the alloy of the present invention is characterized by not including the β-phase in the final state.
In the embodiment of the present invention, the alloy composition has compositions different from the conventional hot-forged TiAl material, and specifically, the alloy element parameter "P = (41 - AI)/3 + 0.25Nb + 0.8Cr - 0.8Si - 1.7C" is in the range of from 1.1 atom% to 2.3 atom%. By this alloy composition, a phase transformation process (α+β → α → α+γ → α2+γ) is realized, which is not realized in the conventional hot forged material, and it is possible to obtain the perfect lamellar structure laminated with the α2-phase and the γ-phase, in which the β-phase is not included in the final state and the high-temperature strength is high, using the phase transformation in the processes of the hot forging and the heat treatment. That is, both of the hot forgeability and the high-temperature strength are balanced. In addition, due to the effect that plastic strain is imparted by the hot forging, the crystal grain becomes finer and thus the room-temperature ductility, the impact properties, and the like are significantly superior to those of the casting material.

[Example]

The present invention will be described below with reference to the accompanying drawings.
Figs. 1(A) to 17 relate to an embodiment of the present invention.
First, preparation procedures and evaluation test procedures of a hot-forged TiAl alloy according to the embodiment of the present invention will be sequentially described in detail.

Procedure 1: Ingot preparation

Figs. 1(A) to 1(C) illustrate an ingot used in Example and a hot forging test for evaluating hot forgeability; Fig. 1(A) illustrates an appearance photograph of the ingot and a cutting position (using a lower side) of a material subjected to a forging test, Fig. 1(B) is a circumstantial photograph during the hot forging test, and Fig. 1(C) is an explanatory view of a change of height in the hot forging test.
Fig. 1(A) is a representative example of the appearance of the ingot prepared by alloy compositions illustrated in Figs. 2 and 3. All of the ingots have almost the same appearance. Figs. 2 and 3 are diagrams illustrating compositions of trial ingots and summaries of evaluation test results of the trial ingots. The ingot is prepared by high-frequency melting using an yttria crucible. A raw material of the ingot includes sponge Ti, granular raw materials of Al, Nb, Cr, and Si, and C added in the form of a TiC powder, and the total weight is about 700 g. A melting atmosphere is in argon gas. Casting was performed using a cast iron mold having an inner diameter of φ 40 mm, cutting is performed at the position illustrated in Fig. 1(A), and the lower side is subjected to the hot forging test. The weight of the ingot in the photograph was about 700 g, but the weight of the ingot after riser cutting was about 450 g.

Procedure 2: Measurement of an area ratio of a β-phase existing at 1350°C (heating temperature during hot forging)

With respect to the ingot prepared in the above procedure 1, a small piece was worked from an upper portion the cut plane of the ingot, and was subjected to a water-cooling treatment after being held at 1350°C for one hour. Subsequently, a cross-sectional structure of the test material subjected to the water-cooling treatment was observed by a reflected electron image of a scanning electron microscope, and the resulting photograph was subjected to an image treatment, whereby the area ratio of the β-phase existing in the test material was measured.

Procedure 3: Hot forging test

The hot forging test was performed in the same manner as the circumstantial photograph illustrated in Fig. 1(B) and the explanatory view illustrated in Fig. 1(C). That is, the heating temperature was 1350°C, the ingot was taken out of the furnace and was placed in a press, and forging was performed by descending of the press. The descending speed of the press was 50 mm/second or faster, the forging direction was upset, and the number of times of the forging was seven times. The material returned to the furnace every each forging and was subjected to reheating. In the hot forging test, the height was changed into 90 mm (initial height of the ingot), 80 mm, 70 mm, 55 mm, 40 mm, 30 mm, 20 mm, and 15 mm, and compression was performed in this order.

Procedure 4: Investigation on presence or absence of β-phase remaining in each composition

After being held at 1350°C for two hours, the hot-forged test material was subjected to an annealing treatment for cooling at 0.2°C/min., and cross-sectional structure thereof was observed by a reflected electron image of the scanning electron microscope, whereby the presence or absence of the β-phase remaining was investigated. This heat treatment was intended to investigate whether the β-phase was ultimately stabilized in each composition of Figs. 2 and 3, and thus the annealing treatment was performed for the purpose. In addition, this heat treatment is independent of heat treatment conditions after the forging which is a requirement of the present invention.

Procedure 5: Investigation of appropriate heat treatment conditions

The hot forged material after the above procedure 3 was subjected to a heat treatment test by changing of the following conditions, and appropriate heat treatment conditions were investigated from structure observation. The changed conditions include a holding temperature, a holding time, and a cooling rate.
As a result, with respect to the alloy of the embodiment according to the present invention, that is, the hot-forged TiAl alloy having an alloy element parameter P (= (41 - AI)/3 + 0.25Nb + 0.8Cr - 0.8Si - 1.7C) in the range of from 1.1 atom% to 2.3 atom%, it was found that the temperature range of the holding temperature for holding the alloy in an equilibrium temperature range of α-single phase region was preferably 1230 to 1290°C.
It was found that the holding time was a time for holding the hot-forged TiAl-based alloy within the equilibrium temperature range of the α-single phase region and was preferably 1 to 20 hours.
It was found that the cooling rate was a cooling rate of the alloy after the hot-forged TiAl-based alloy was held in the equilibrium temperature range of the α-single phase region for a predetermined time, and was preferably 1 to 10 [°C/min.].
Subsequently, in the Procedure 5 of Investigation of appropriate heat treatment conditions, an appropriate structure is determined as follows. That is, an object of structure is a fine structure in which lamella grains are densely arranged, the lamella grains being alternately laminated with an α2-phase of gray in the reflected electron image and a γ-phase of black in the reflected electron image and having an average grain size of 1 to 200 µm. In addition, a β-phase of white in the reflected electron image or a γ-grain in which the equi-axied γ-phase of black in the reflected electron image is largely grown is not included. Silicide of a small white granular shape in the reflected electron image is outside the scope of the evaluation determination, the silicide being precipitated along with the addition of Si.

Procedure 6: Evaluation creep rupture strength

After the hot forged material was subjected to the heat treatment, a creep test piece was worked and was subjected to a creep rupture test in a state of 870°C × 225 MPa. Then, creep strength of each alloy was evaluated by a rupture time. The inventive alloy was subjected to the heat treatment under heat treatment conditions to obtain the object of structure in the procedure 5. Further, Comparative Alloys (alloys in which the β-phase remains in the procedure 4) is treated under the appropriate conditions in the inventive alloy having an analogous composition.
Fig. 4 is an explanatory view illustrating a relation between an alloy element parameter "P = (41 - AI)/3 + 0.25Nb + 0.8Cr - 0.8Si - 1.7C" of a trial ingot of the present invention and a forging test result at 1350°C measured in the procedure 3 and a relation between the area ratio of the β-phase of a material, which is water-cooled in the condition of 1350°C × 1 h, measured in the above procedure 2 and the forging test result. In Fig. 4, each plot corresponds to a separate ingot having a different composition, and a state of crack occurrence in the hot forging is indicated by a black-plotted mark or a void-plotted mark. During the hot forging test, the crack occurs in the case of the ingot having a composition of the black-plotted mark, and the crack does not occur in the case of the ingot having a composition of the void-potted mark.
From Fig. 4, it can be confirmed that the correlation between the alloy element parameter "P = (41 -AI)/3 + 0.25Nb + 0.8Cr - 0.8Si - 1.7C" and the area ratio of the β-phase of the material which is water-cooled in the condition of 1350°C × 1 h is good. In addition, the relation between the hot forgeability and the area ratio of the β-phase of the material which is water-cooled in the condition of 1350°C × 1 h and the relation between the alloy element parameter P and the area ratio of the β-phase of the material which is water-cooled in the condition of 1350°C × 1 h are as follows. An ingot having a composition in which the alloy element parameter P is 1.1 atom% or less and the area ratio of the β-phase of the material which is water-cooled in the condition of 1350°C × 1 h is 30% or less has poor hot forgeability. On the other hand, an ingot having a composition in which the alloy element parameter P is 1.1 atom% or more and the area ratio of the β-phase of the material which is water-cooled in the condition of 1350°C × 1 h is 30% or more has excellent hot forgeability.
Fig. 5 is an explanatory view illustrating a relation between an alloy element parameter "P = (41 - AI)/3 + 0.25Nb + 0.8Cr - 0.8Si - 1.7C" of the trial ingot of the present invention and the presence or absence of the β-phase residue in an annealing treatment evaluated in the procedure 4 (whether the β-phase is finally stable in each composition) and a relation between the area ratio of the β-phase of a material, which is water-cooled in the condition of 1350°C × 1 h, measured in the above procedure 2 and the presence or absence of the β-phase residue.
The relation between the presence or absence of the β-phase residue and the alloy element parameter P and the relation between the presence or absence of the β-phase residue and the area ratio of the β-phase of a material, which is water-cooled in the condition of 1350°C × 1 h, are as follows. In an ingot having a composition in which the alloy element parameter P is 2.3 atom% or less and the area ratio of the β-phase of the material which is water-cooled in the condition of 1350°C × 1 h is 60% or less, the β is eliminated after the annealing treatment. That is, in this composition, the β-phase is finally unstable. On the other hand, in an ingot having a composition in which the alloy element parameter P is 2.3 atom% or more and the area ratio of the β-phase of the material which is water-cooled in the condition of 1350°C × 1 h is 60% or more, the β remains after the annealing treatment. That is, in this composition, the β-phase is finally stable.
From the above results illustrated in Figs. 4 and 5, it is possible to evaluate the hot forgeability and the influence of the alloy composition on the stability of the final β-phase using the alloy element parameter "P = (41 - AI)/3 + 0.25Nb + 0.8Cr - 0.8Si - 1.7C". It could be confirmed that the hot forgeability was excellent and the β-phase did not finally remain when the parameter was in the range of from 1.1 atom% to 2.3 atom%.
The hot forged materials of the ingots prepared by the compositions illustrated in Figs. 2 and 3 will be described in detail below based on typical cases by being divided into Examples and Comparative Examples.

[Example 1]

Fig. 6 is an appearance photograph when an ingot (alloy 13 having a composition of Ti-42Al-8Nb-2.3Cr-0.9Si-0.7C (atom%)) according to the embodiment of the present invention is subjected to the hot forging at 1350°C. Since it is estimated that the amount of β-phase at 1350°C is 42% much larger than that in the evaluation in the procedure 2, forgeability is good, and no crack occurs.
Fig. 7 is a reflected electron image photograph of a test material obtained in such a manner that the ingot (alloy 13) according to the embodiment of the present invention is heat-treated under appropriate conditions after being subjected to the hot forging. A perfect lamellar structure having no β-phase (large white phase) appears in the photograph. Fine white points indicate precipitates (silicide) caused by Si. Here, the appropriate conditions refer to the heat-treatment conditions described above.
That is, when the alloy 13 subjected to the hot forging is heat-treated under the appropriate conditions, the β-phase existing in the hot forged material is no longer present in the alloy, the β-phase having excellent high temperature deformability (low high-temperature strength). The grain size is slightly coarsened compared to that of the forged alloy, but becomes significantly smaller than that of a casting material. Therefore, since this hot forged material has the above structure, it is excellent in both of the high-temperature strength and the room-temperature ductility.

[Comparative Example 1]

Fig. 8 is an appearance photograph when an ingot (composition: Ti-41AI-7Nb-0.9Si-0.4C (atom%)) of Comparative Alloy 6 is subjected to the hot forging at 1350°C. Since it is estimated that the amount of β-phase at 1350°C is 12% smaller than that in the evaluation in the procedure 2, deformability is poor, and large cracks have occurred.
Fig. 9 is a photograph of a reflected electron image structure of a test material obtained in such a manner that the forged TiAl material of Comparative Alloy 6 is heat-treated under appropriate conditions. Similarly to the inventive alloy, a perfect lamellar structure having no β-phase (large white phase) appears in the photograph. Fine white points indicate precipitates (silicide) caused by Si.

[Comparative Example 2]

Fig. 10 is an appearance photograph when an ingot (composition: Ti-40AI-7Nb-3Cr-0.6Si-0.9C (atom%)) of Comparative Alloy 4 is subjected to the hot forging at 1350°C. Since it is estimated that the amount of β-phase at 1350°C is 63% much larger than that in the evaluation in the procedure 2, forgeability is good, and no crack occurs.
Fig. 11 is a photograph of a reflected electron image structure of a test material obtained in such a manner that the ingot of Comparative Alloy 4 is heat-treated under appropriate conditions after being subjected to the hot forging. Since a β-phase (large white phase) having excellent high temperature deformability (low high-temperature strength) remains, it is assumed that the high-temperature strength is low. In fact, a creep rupture time (h) in a state of 870°C × 225 MPa is 16 hours which is shorter than that in the inventive alloy.

[Comparative Example 3]

Fig. 12 is a reflected electron image photograph of a test material obtained in such a manner that the ingot (alloy 13) according to the embodiment of the present invention is held at 1220°C lower than the appropriate holding temperature in a heat treatment after being subjected to the hot forging. Other heat treatment conditions are appropriate conditions. It is found that a large black equi-axied γ-phase exists. That is, since a perfect lamellar structure is not formed, it is considered that the high-temperature strength is lower than that of the inventive alloy. This is considered because the holding temperature of 1220°C is within a (α+γ) region rather than an α-single phase region.

[Comparative Example 4]

Fig. 13 is a reflected electron image photograph of a test material obtained in such a manner that the ingot (alloy 13) according to the embodiment of the present invention is held at 1300°C higher than the appropriate holding temperature in a heat treatment after being subjected to the hot forging. Other heat treatment conditions are appropriate conditions. It is found that a large white β-phase exists. Since the β-phase remains, it is considered that the high-temperature strength is lower than that of the inventive alloy. This is considered because the holding temperature of 1300°C is within a (α+β) region rather than an α-single phase region.

[Comparative Example 5]

Fig. 14 is a reflected electron image photograph of a test material obtained in such a manner that the ingot (alloy 13) according to the embodiment of the present invention is held for 0.5 hours shorter than the appropriate holding time in a heat treatment after being subjected to the hot forging. Other heat treatment conditions are appropriate conditions. It is found that a large white β-phase exists. Since the β-phase remains, it is considered that the high-temperature strength is lower than that of the inventive alloy. This is considered because the holding time is short and thus a sufficient time for transformation of the β-phase existing in the forged material into the α-phase is not left.

[Comparative Example 6]

Fig. 15 is a reflected electron image photograph of a test material obtained in such a manner that the ingot (alloy 13) according to the embodiment of the present invention is held for 23 hours longer than the appropriate holding time in a heat treatment after being subjected to the hot forging. Other heat treatment conditions are appropriate conditions. It is found that a perfect lamellar structure is formed, but a crystal grain is large. Since the crystal grain is large, it is considered that the room-temperature ductility or the like is lower than that of the inventive alloy. This is considered because the holding time is long and thus an α-grain (lamellar grain after cooling) is coarsened during the holding.

[Comparative Example 7]

Fig. 16 is a reflected electron image photograph of a test material obtained in such a manner that the ingot (alloy 13) according to the embodiment of the present invention is cooled at 0.7 [°C/min.] slower than the appropriate cooling rate in a heat treatment after being subjected to the hot forging. Other heat treatment conditions are appropriate conditions. It is found that a perfect lamellar structure is formed, but a lamella gap is large. Since the lamella gap is large, it is considered that the high-temperature strength is lower than that of the inventive alloy.

[Comparative Example 8]

Fig. 17 is a reflected electron image photograph of a test material obtained in such a manner that the ingot (alloy 13) according to the embodiment of the present invention is cooled at 15 [°C/min.] faster than the appropriate cooling rate in a heat treatment after being subjected to the hot forging. Other heat treatment conditions are appropriate conditions. It is found that a perfect lamellar structure is formed, but a lamella gap is small. Since the lamella gap is small, it is considered that the room-temperature ductility or the like is lower than that of the inventive alloy.
The above embodiments are merely made to describe in detail the present invention. Accordingly, the present invention should not be restrictively construed with the above embodiments. The TiAl-based alloy of the present invention or the method for producing the TiAl-based alloy includes ratio changes of composition elements within an obvious range in a person skilled in the art, for example, composition changes in an allowable range included inevitably in manufacturing or composition changes in an allowable range depending on variations in purchase price or fluctuations in supply state of raw-material compositions.

Industrial Applicability

The TiAl-based alloy according to the present invention is excellent in high-temperature strength or impact resistance, and thus is suitably used for a rotor blade of a gas turbine or steam turbine for power generation, aircraft, ship, or various industrial machines.
The TiAl-based alloy material produced by the present invention is excellent in high-temperature strength and has excellent ductility or impact properties. When this material is used for the rotor blade of various turbines or turbocharger, it is possible to improve energy efficiency due to an increase in an engine speed and contribute to reduction in weight while maintaining reliability.
In addition, the TiAl-based alloy according to the present invention can be used to manufacture large parts from excellent hot forgeability and is suitably used for the rotor blade or a disk of an aircraft engine or the gas turbine for power generation because of being excellent in high-temperature strength, room-temperature ductility, or the like.
In the case of using the TiAl-based alloy according to the present invention, it is possible to obtain a large-scaled material which is excellent in high-temperature strength and room-temperature ductility. Since the rotor blade or disk made of this material has excellent high-temperature strength or room-temperature ductility, when this material is used for the rotor blade of the aircraft engine or the gas turbine for power generation, it is possible to improve energy efficiency due to an increase in an engine speed and an increase in size of parts while maintaining reliability.

Claims

A TiAl-based alloy, consisting of:
Al: 41 to 45 atom%,

Nb: 7 to 9 atom%,

Cr: 0.4 to 4.0 atom%,

Si: 0.3 to 1.0 atom%, and

C: 0.3 to 1.0 atom%,

and optionally 0.1 to 3 atom% total of at least one element selected from the group consisting of W, Mo, B, Hf, Ta and Zr, with the balance being Ti and inevitable impurities,

wherein an alloy element parameter P obtained by the following formula is in the composition range of from 1.1 to 2.3; $P = (41 - Al) / 3 + 0.25 Nb + 0.8 Cr - 0.8 Si - 1.7 C,$
and

the TiAl-based alloy has a fine structure in which lamella grains laminated alternately with a Ti₃Al phase (α2-phase) and a TiAl phase (γ-phase) are densely arranged and a β-phase is not included, the lamella grains having an average grain size of 1 to 200 µm.
A method for producing a TiAl-based alloy, comprising:
a process in which the TiAl-based alloy according to claim 1 is held at a coexisting temperature range of a hexagonal close-packed structure phase (α-phase) and a body-centered cubic structure phase (β-phase) and is then subjected to hot forging; and

a process in which the hot-forged TiAl-based alloy material is held in a temperature range of from 1230°C to 1290°C for 1 to 20 hours and is then subjected to a cooling treatment at a rate of from 1 [°C/min.] to 10 [°C/min.].
A rotor blade for a turbine that uses the TiAl-based alloy material produced in such a manner that an ingot having the composition according to claim 1 is produced by the production method according to claim 2.
A turbine that uses the rotor blade for turbine according to claim 3.